1. Field of the Invention
This invention relates to an innovative connection between cooled pipe and uncooled pipe in a double-pipe heat exchanger of the type used, for example, to cool high temperature cracking gas in so-called Transfer Line Exchangers (TLEs).
2. State of the Prior Art
In these exchangers the double-wall pipe comprises an internal pipe traveled by the fluid to be cooled (for example, cracking gas coming out of an oven) and an external pipe delimiting with the internal one the air-space traveled by the cooling fluid (for example, water) with the cooling fluid injected into the air space through a union on the side wall of the outermost pipe and in general near the inlet end of the double-wall cooled pipe. The cooling fluid is then taken from the air space near the output end of the double-wall pipe.
The double-wall pipe must be connected upstream with a single-wall pipe inlet pipe feeding the hot fluid to be cooled and which is at a relatively high temperature. It is to be considered for example, that, in ethylene plants, the incoming hot fluid has a temperature over 800° C.
In the field of heat exchangers of this type the problems had at the connection between the cooled double-wall pipe and the inlet pipe of the fluid to be cooled are well known.
To realize the union, it has been proposed to use a forked connection having on one side two shanks designed for the connection with the two pipes of the double-wall pipe and, on the other side, a shank stretching longitudinally to be jointed with the inlet pipe of the fluid to be cooled.
During operation the forked connection is subjected to a considerable thermal stress due to the high temperature reached by its shank which is connected to the hot-fluid inlet pipe.
To avoid the fork in the long run being damaged because of excessive thermal stress, it has been proposed to insert at the height of the fork in the duct of the fluid to be cooled a transition cone which would by-pass the critical portion of the connection and cause the gasses in temperature downstream of the fork and already inside the double-wall pipe to flow.
These solutions complicate not a little the design of the exchanger by forcing insertion of the supplementary cone and, possibly, even a refractory ring between the cone and the fork to improve distribution of the temperature at the fork.
In addition, the gas to be cooled meets on its path irregularities which disturb the gas flow and cause formation of coke in the apparatus. The irregularity consists of the floating ‘sleeve’ arranged generally at the inlet of the double pipe and capable of absorbing the differential dilation between the outer wall of the cone in contact with the air and the inner wall of the pipe in contact with the hot gas. The coke, by attrition, obstructs dilations of the ‘sleeve’ which occur at each startup of the exchanger and compromise mechanical integrity. In addition, coke formation causes fouling and decrease in the efficiency of the exchanger.
Lastly, lengthening of the geometry causes an increase in the gas stay time in the exchanger with resulting worsening of the product final output.
The general purpose of this invention is to remedy the above-mentioned shortcomings by making available a connection for a heat exchanger with double-wall pipes and having a simple, economical structure and which at the same time is durable and resistant to the operating temperatures of the exchanger in every part thereof.
Another purpose of this invention is to make available a connection for the double-pipe heat exchanger allowing avoidance of the formation of coke as well as high efficiency.